Exoskeleton system for reinforcing tall buildings

ABSTRACT

A system for reinforcing tall buildings. The building comprises an internal frame comprising a plurality of horizontal floor beams and a plurality of vertical columns extending between adjacent floor beams. The reinforcing system comprises diagonal bracing members connected between vertically spaced floor beams at an angle to both the floor beams and the columns.

INTRODUCTION

[0001] The author presents a new vertical bracing system, placed outsidethe building envelope, for concrete or steel framed buildings whichprovides excellent resistance to seismic forces in high rise buildingsat a cost expected to be lower than conventional framing systems.

[0002] The proposed system is a combination of simple standard boltedconnections (shear connections) for the building frame and verticalbracing located approximately 2′6″ (≅75 cm) outside the face of thebuilding framing. The framing supporting the vertical loads (buildingcolumn and beams) is separate from the lateral load bracing (outsidevertical bracing).

[0003] The outside vertical bracing system has excellent ductility underelastic load reversals (i.e. wind and moderate seismic forces) due tothe elastic hinges developed between the outside bracing and thebuilding framing.

[0004] Under inelastic load reversals (i.e. severe seismic forces) theelastic hinges have the potential for becoming plastic, thus providingeven greater ductility. The elastic and plastic hinges act as structuralfuses protecting the framing.

[0005] Steel high rise buildings which must be designed for horizontalloads (seismic forces or wind forces) require moment connections orvertical bracings. For very tall buildings, the trend today is to usefield welded framing where large assemblies, which form the exterior ofthe building, are fabricated in the shop then are shipped to the sitewhere they are field welded. The use of field welded framing and verylarge built-up members (especially box sections) requires more material,takes more time, and makes the building more expensive. In addition itsometimes makes the building more flexible, creating problems with theoutside curtain wall, glass panels, etc.

[0006] Description of Framing System

[0007] Following is a description of the proposed system of outsidevertical bracing. The vertical bracing mounted outside the face of thebuilding framing consists of diagonal bracing, and horizontal andvertical struts (see FIGS. 1 and 2). The columns of the standard framingof the building will carry the vertical forces due to overturning fromhorizontal forces (wind or seismic). Therefore, the construction of thebuilding can proceed in the same manner as framing with standardconnections (shear connections: angles shop welded to beams and fieldbolted to columns). As the building progresses in height, another teamcan install the outside vertical bracing (diagonal bracing andhorizontal and vertical struts only). Thus the vertical bracing lagsbehind the main steel framing and does not interfere with the regularschedule of steel erection for standard connection framing. Becausefield welding is minimized and the framing of the building requiresrolled shapes only, the construction time is reduced and so the price ofthe building is reduced.

[0008] Diaphragm Floors

[0009] The floors attached by stub connectors to the outside horizontalstrut are held in place by the bracing and are designed as diaphragmfloors.

[0010] All the columns (interior and exterior) between two diaphragmfloors have about the same absolute lateral stiffness, K=P. As a result,the seismic forces transferred by all the columns are about equal. Thediaphragm floor collects tributary seismic forces from the floors aboveand below and also resists the seismic forces due to its own dead load(concrete, partitions, steel framing, curtain wall, etc.). The diaphragmfloor transfers the seismic forces to the outside horizontal strutthrough the stub connectors, as previously described.

[0011] Stub Connectors

[0012] The stub connectors attaching the outside vertical bracing to thebuilding act like damping springs or structural fuses which absorbenergy due to seismic forces. They deliver forces due to overturning ofthe building by bending and shear to the building columns and transferthe horizontal forces by bending about their weak axis. One can“tune-up” this damping system by designing the stub connectors to bemore rigid or more elastic and to yield at a certain value of lateralforces. After an earthquake, all the stub connectors can be inspectedand the damaged ones replaced. Therefore, when detailing the stubconnectors, it is necessary to provide for possible replacement by usingbolted plates in addition to shop welding. The “tune-up” of the stubconnectors can be done using a model of the building.

[0013] Sub-Frames

[0014] Due to the action of elastic hinges, the building frame isdivided into two sub-frames. Sub-frame 1 consists of all of thediaphragm floors and the outside vertical bracing which are connectedthrough stub connectors. Sub-frame 2 consists of all of the floorssupported between the diaphragm floors and all of the columns of thebuilding (interior and exterior). Sub-frame 1 and sub-frame 2 areconnected with elastic hinges which can develop into “plastic hinges”,insuring ductility in response to a wide range of load intensities.

[0015] Damping Action of Framing System

[0016] Due to the elastic hinges, located at all the floors, andsub-frames, there is a substantial damping action which will absorb alarge quantity of seismic energy. In order to increase the damping, mildcarbon steel ASTM A-53 Grade B (Fy=25. KSI to Fy=35 KSI) can be used forstub connectors (Elastic Hinge N 1) and for the column splice (ElasticHinge n 3). See FIG. 4.

[0017] In addition, there is damping due to the different modes ofdeflection and to the different fundamental elastic periods of vibrationbetween the columns (interior and exterior) and the outside verticalbracing, in the direction under consideration. The damping provided bythe outside vertical bracing must be established by extensive laboratorytesting on a model.

[0018] Building codes use the factor K which defines the ductility ofthe system framing and also reflects the redundancy of the system(framing).

[0019] For highly ductile and redundant frames which are the rigidmoment frames, the code identifies a value of K=0.67. The code does notallow smaller values for K.

[0020] For systems which are more “brittle” and do not have largeinelastic deflections, the value of K is very large (K=1.33) becausethese systems cannot reduce the vibrations and the resulting deflectionscaused by the earthquake.

[0021] The system of outside vertical bracing is a system which fitsbetween a “brittle” system and a highly ductile and redundant system dueto the elastic hinges, plastic hinges and the sub-frames.

[0022] We propose a formula for K as follows:$K = {{0.67 +} \propto \frac{0.33}{n}}$

[0023] In this formula, “n” is the number of stories between thediaphragm floors. The true value of K must be determined by modeling.

[0024] To quantify the proposed system, the computer program HERCULE,created by the French Control Office SOCOTEC-PARIS, was used to model athirty story building 10′ floor heights, with three 15′ bays in eachdirection, see Supplemental I for more details. Three cases were studiedand compared:

[0025] A. The horizontal and vertical load framing systems in the sameplane as the building exterior.

[0026] B. The outside lateral load bracing placed approximately 2′6″(≅75 cm) outside the building envelope and linked to the interiorstructure by rigid connections on the extension of the main canteleverbeams.

[0027] C. Configuration identical with Case B, with the addition ofanti-seismic springs between the two structures.

[0028] Computer analysis indicates that deformations are quite similarin Cases A and B due to the great rigidity of the cantilever beams.However, the maximum deflections are reduced by 2.5 times in Case C dueto the springs or elastic hinges. The sway of the inner structureincreased from 7 to 16 cm in Case C over the results in Cases A and B,but still remains within acceptable limits for a building 300′ high,FIG. 1A.

[0029] In addition, a 60% to 70% decrease in the axial stress in thecolumns and main bracing members was realized. The corner columns of theinner structure had a 75% decrease in axial stress, but was accompaniedby a parallel increase in the maximum flexural moments. The increase inflexural moments was forseen due to the loading arrangements and can bemitigated by introducing hinges at the bottom of the columns instead offixing them. Finally, the reactions in the anti-seismic springs variesfrom 5 to 12T (max.) which is in the range of springs commonly used.

[0030] {circle over (5)} Interior Diagonal Bracing

[0031] The interior diagonal bracing is made up of steel members runningdiagonally between the corners of the floor modules.

[0032] To achieve greater flexibility for architectural aspects, thesediagonal members can extend one or multiple floors in height. Thesemembers consist of a wide flange section at the building center fornarrow structures or at ¼ or ⅛, etc. panels for wider and/or tallerbuildings.

[0033] Interior bracing can be 2 or 4 floors in height and shouldcomplete the standard triangle shape:

[0034] Note: Exterior bracing can be any of the above configurations.

[0035]FIGS. 3A and 3B

[0036] System Advantages

[0037] The framing system described in this paper has the followingadvantages over conventional high rise design:

[0038] 1. The lateral bracing is approximately 2′6″ (≅75 cm) outside thebuilding exterior so that multiple floors (three or four maximum) can bebraced together thus eliminating many connections.

[0039] 2. The structural steel frame and exterior bracing can be boltedmoment connections for greater economy.

[0040] 3. The design of all but the elastic hinge stub connectors can beperformed with desk top calculators.

[0041] 4. The framing has a built in self damping system due to theaction of the stub connectors which connect the building framing withthe outside vertical bracing.

[0042] 5. The vertical bracing is outside the building so it can beeasily inspected for damage after an earthquake; repair work is alsosimplified.

[0043] 6. The bracing system lends itself to retrofitting existingconcrete or steel buildings which do not meet present seismic codes.

[0044] 7. The framing system could allow for very tall structures up toapproximately 115 floors.

[0045] Work-in-Progress

[0046] The author is in the process of evaluating, by computer modeling,three additional and different cases for the exterior bracing systems.All three cases have the same geometric characteristics and allincorporate diaphragm floor connections at the top and bottom of eachexterior brace, the difference is in the relative strength of the twosupport systems. The two systems are described below:

[0047] System A:

[0048] This framing systems consists of the exterior building columns,the two diagonals of the triangular bracing and the exterior floor beamsto which the triangular bracing is attached.

[0049] System B:

[0050] This framing system consists of the two horizontal and twovertical members of the outside bracing as well as the same component ofthe diagonal outside braces which it shares with System A.

[0051] By varying the relative strengths of the two systems, the authorcan evaluate the most effective use of the bracing. The three casesunder consideration are:

[0052] {circle over (1.)} Case 1:

[0053] System A resists 90% of the load

[0054] System B resists $\frac{10\%}{100\%}$

[0055] of the load

[0056] {circle over (2.)} Case 2:

[0057] System A resists 50% of the load

[0058] System B resists $\frac{50\%}{100\%}$

[0059] of the load

[0060] {circle over (3.)} Case 3:

[0061] System A resists 5% of the load

[0062] System B resists $\frac{95\%}{100\%}$

[0063] of the load

[0064] See FIGS. 1 through 5

[0065] {circle over (4.)} Supplement I—Computer Simulation

[0066] In order to give an example and qualify our proposal, we used thecomputer program called HERCULE that has been tested by the FrenchControl Office SOCOTEC-PARIS.

[0067] We have considered the spectrum of horizontal response of USNECshown on FIG. 8.

[0068] We studied a square building with three bays of 7 m in bothdirections and of the height of 30 levels 3 m each.

[0069] We considered:

[0070] A. The interior structure for the vertical leading schedule (FIG.9) with beam data as follows: A = 135 cm2 S = 2500 cm3 I = 67500 cm4

[0071] Columns with varying characteristics every 3 levels: Level 28 to30 A = 125 cm2 S = 2500 cm3 I = 75000 cm4 25 to 27 250 5000 150000 22 to24 375 7500 225000 19 to 21 500 10000 300000 16 to 18 625 12500 37500013 to 15 750 15000 450000 10 to 12 875 17500 525000 7 to 9 1000 20000600000 4 to 6 1125 22500 675000 1 to 3 1250 25000 750000

[0072] B. The exterior structure for horizontal stability (FIG. 10) withcolumns identical to the inner structure and cross bars changing sectionevery 6 levels: Level 25 to 30 A = 30 cm2 S = 600 cm3 I = 20000 cm4 19to 24 60 1200 40000 13 to 18 90 1800 60000  7 to 12 120 2400 80000 1 to6 150 3000 100000

[0073] We studied and compared 3 cases:

[0074] A. Both structures on the same plane surface of the facade actingintegrally.

[0075] B. The structure for horizontal stability placed 80 cm in frontof the interior structure and linked to this one by hinged connectionson the extension of the main cantilever beams.

[0076] C. Identical to Case B with addition of 14 anti-seismic springson each facade as shown on FIG. 11.

[0077] The results are presented on FIGS. 12 to 16. We have representedon FIG. 12 the deflection of the horizontal stability structure in allthree cases.

[0078] We noticed that deformations are quite similar in Cases A and Bdue to the great rigidity of the cantilever beams. But in Case C due tothe springs, the maximum deflection is reduced from 7.2 cm to 2.87 cm,2.5 times less.

[0079] In addition we notice on FIG. 13 that the maximum sway of theinner structure (practically the same for Cases A and B) is increasedfrom 7.26 and 7.76 cm to 16.37 cm.

[0080] We observe, however, that a lateral displacement of 16 cm for a90 m tall building is acceptable (1.7%) and that the difference in swaybetween the 2 structures 16.37−2.87=13.50 cm must be in agreement withthe anti seismic springs.

[0081] On FIG. 14, we have indicated the stresses in the columns and inthe bracing of the horizontal stability structure.

[0082] We notice for the most loaded columns a decrease of:

[0083] 367 t in Case A

[0084] 342 t in Case B (−7%)

[0085] 157 t in Case C (−58%)

[0086] and for the most loaded bracings a decrease of:

[0087] 234 t in Case A

[0088] 226 t in Case B (−4%)

[0089] 80 t in Case C (−66%)

[0090] On FIG. 15, we have shown the axial stresses and the flexuremoments for the corner columns of the inner structure.

[0091] We observe for the most loaded columns a decrease of:

[0092] 365 t in Case A

[0093] 240 t in Case B (−35%)

[0094] 99 t in Case C (−73%)

[0095] accompanied by a parallel increase of the maximum flexuremoments. The increase of the flexural moments is compatible with thepresence of the axial loads due to the loading schedule and thisincrease can be limited by introducing hinges at the column bottoms onfoundation instead of the fixed ends considered in the study.

[0096] Finally on FIG. 16, we have indicated the value of the reactionsin the antiseismic springs (varying from 5 T to 11,6 T maximum) valuesin agreement with the mechanical characteristics of the springs commonlyused in practice.

[0097] We have tried on FIGS. 17 to 19 to sketch three architecturalsolutions among others possible.

[0098] In conclusion, it is the author's belief that the use of theoutside bracing for tall buildings will achieve economy of materials anddown the construction time.

[0099] {circle over (6.)} Supplement II—Exterior Bracing for ReinforcedConcrete Structures

[0100] Framing of structures in a seismic area must be able to resistthe earthquake without collapsing and with minimum damages.

[0101] A steel framing is well suited for earthquake because it isflexible and can absorb the energy from the earthquake.

[0102] After displacement, the steel framing comes back in almostt thesame initial position.

[0103] Concrete framings, because they are more rigid (brittleframings), have a disadvantage in absorbing the energy from earthquakeand regarding the displacment.

[0104] Present building codes require for rigid ductile concrete framesa special design and detailing which provides sufficient reinforcementto ensure ductile frame behavior.

[0105] The trouble is that once the concrete is poured, all thistremendous amount of reinforcement is “locked” in the mass of concretewhich is brittle.

[0106] When the earthquake acts on the building, the concrete is“activated” first and the concrete framing has a brittle behavior.

[0107] When the level of this brittle behavior is exceeded and theconcrete has suffered excessive (rotations, displacement, cracks), thenand only then is the steel reinforcement “activated” to full capacity.

[0108] It is my conclusion from all the data in respect to effects ofearthquake on concrete buildings that there is no harmonious responsefrom concrete and reinforcement in resisting the earthquake due to thefollowing behavior:

[0109] When the earthquake acts on the reinforced concrete building, theconcrete is “activated” first. The reinforcement is “locked” in the massof concrete. Therefore, we have a brittle behavior which can cause verysevere damages or collapse of the building.

[0110] Usually the reinforced concrete building has a concrete shearwall which must resist all of the earthquake force and a rigid framewhich is the second line of defense.

[0111] The rigid frames (the second line of defense) must resist theportion of the earthquake which is absorbed by the rigid framesproportional to their absolute lateral stiffness versus the absolutelateral stiffness of the concrete shear walls. However, the coderequires that the rigid frame absorbs minimum 25% of the totalearthquake force.

[0112] Therefore, the present approach for the reinforced concretebuilding to resist seismic forces starts with a rigid frame and shearwalls, which essentially is a brittle structure.

[0113] The follows a theoretical approach which, using a tremendousseries of assumptions in the design and detailing of the reinforcedconcrete, tries to transform the brittle structure into a rigid ductileconcrete frame.

[0114] In the last few years a new theoretical approach was developed:the dynamic inelastic design approach. This design starts from the sameconcrete framing, which is a brittle structure. Then, using thisinelastic design approach, the brittle structure is transformed into aframing with yielding at certain locations, which means the structure isbehaving like an elastic system capable of developing yielding inselected locations. The inelastic dynamic analysis approach is basedalso on a tremendous series of assumptions and techniques.

[0115] Let's list some of the series of assumptions and techniques:

[0116] No. 1: Earthquake Accelerogram—This is the model of the motion ofthe ground at certain locations where the structure is built. This modelcan be selected as site-dependent or site-independent ground motions. Weknow that the site for a structure us very complex and is not completelyknown. Therefore, the ground motions are very complex and variable.

[0117] No. 2: The Dynamic Inelastic (Response History) Analysis of astructure is a step-by-step investigation, in very small timeincrements, of the response of a given structure to an earthquakeaccelerogram selected as mentioned in No. 1 above. The results of thisdynamic inelastic (response history) analysis are therefore dependent onthe earthquake accelerogram and how accurate is that earthquakeaccelerogram. (This is like a chain reaction.)

[0118] No. 3: Selection of a Hypothetical “Maximum Credible”Earthquake—This is the most severe ground motion which we assume willever happen at the site of the structure. In case we have an earthquakewhich is more severe than the “maximum credible” earthquake selected,the dynamic inelastic analysis is not correct anymore.

[0119] No. 4: Selection of Inelasticity Only on the Horizontal Elements(yielding of beams at certain “hinge” locations)—This selection iscoupled with definite limits on the corresponding “inelasticdeformations.”

[0120] In my opinion, this item can be the subject of two theoreticalresearch, but I don't think we can relate it to the real world ofreinforced concrete structures.

[0121] I want to make the following analogy:

[0122] Elevators, which are moving vehicles to transport people within abuilding, are designed with a factor of safety of F.S.=5.0 (the wireropes and the supporting frame).

[0123] During an earthquake, the reinforced concrete building startsshaking and moving and becomes practically an “elevator” carrying thepeople in the building. The ultimate strength design for reinforcedconcrete provides a factor of safety of F.S.=1.7÷1.8.

[0124] This factor is already a small factor of safety for a “movingvehicle.” To assume that inelasticity will occur only at selectedlocations (horizontal elements which are beams) means to further reducethis factor of safety because we narrow down the behavior of the framingto an assumed theoretical pattern.

[0125] I think this is not acceptable from the point of view of safetyof the people who are in the building. I also think this is not inaccordance with the practice in the construction industry for the simplereason that we cannot push the real frame of reinforced concrete all theway to a condition when the real yielding and hinges will occur. Thisyielding and hinges conditions is only a hypothetical condition and wereally don't know how the concrete building will behave, no matter howmuch computer time and how many iterations we use.

[0126] No. 5: For the Inelastic Dynamic Analysis, the columns and sheerwalls are kept elastic throughout their seismic response.

[0127] It is my feeling that we can use such an approach when we aredealing with wind loads, which are loads with a “mild” behavior andintensity.

[0128] In my opinion, it is wrong to apply this “mild” behavior andintensity to earthquake forces and to assume that columns and shearwalls will stay “elastic” during earthquakes, when they are brittlemembers to begin with.

[0129] No. 6: The Dynamic Inelastic Analysis is greatly affected by themagnitude of the percentage of critical damping assumed. In selectingthe percentage of critical damping, we have to make again anotherassumption.

[0130] I will stop here the long list of items included in the series ofassumptions and techniques used in the present approaches for the designof reinforced concrete structures for earthquake forces. I consider thatthe present approaches are very theoretical and unrealistic. Therefore,we can have very severe damages or collapse of the reinforced concreteframing during a very severe earthquake. These present approaches can besummarized as follows: Starting with a brittle structure and then usinga very theoretical, complex and special design and special detailing,this brittle structure is transformed into a ductile frame behavior orinto an inelastic frame with confined inelasticity only to thehorizontal elements (yielding of beams at certain “hinge” locations).

[0131] In order to provide a reinforced concrete structure which canwithstand satisfactory and more realistic the earthquake, I am proposinga system which is a combination of reinforced concrete rigid frame andvertical steel bracing located outside the face of the reinforcedconcrete building (see FIGS. 6A through 7).

[0132] In this proposed system, the reinforced concrete rigid frames aredivorced from the outside vertical steel bracings, thus permitting thetwo framings to be installed independent from one another, usingconventional methods of construction (rigid frames for reinforcedconcrete, bolted connection for the outside vertical steel bracings).

[0133] The outside vertical steel bracings is the third line of defense(life saver).

[0134] The proposed system has the following advantages:

[0135] (1) Because the outside vertical steel bracings resist a majorpart of earthquake force during the “elastic behavior” of the hybridsystem, and because the outside vertical steel bracings must resist allthe earthquake forces at the “failure stage,” the reinforced concreterigid frames are not very congested with reinforcement, and detailingand construction are simplified.

[0136] (2) When the earthquake acts on the building, both systems (thereinforced concrete rigid frames and the outside vertical steelbracings) are activated and act in a harmonious response.

[0137] The reason for this harmonious response is due to the fact thatwe took out from the concrete a major part of reinforcement which waslocked in the mass of brittle concrete and then we converted thisreinforcement into outside vertical steel bracings which actindependently.

[0138] Thus, when the earthquake acts, the reinforced concrete rigidframes and the outside vertical steel bracings act simultaneously andresist part of the earthquake forces proportional to their absolutelateral stiffness. This advantage is one of the most importantadvantages.

[0139] (3) After an earthquake, a reinforced concrete building hascracks and minor and major damages. It is very hard to evaluate if thebuilding can resist another severe earthquake.

[0140] Repairs done to a reinforced concrete framing are veryquestionable as far as their strength and efficiency for the nextearthquake.

[0141] With the proposed method, after an earthquake the outsidevertical steel bracings can be inspected and parts which are damaged canbe replaced or reinforced.

[0142] The reinforced concrete frames can also be repaired. The strengthand the efficiency of the reinforced concrete building is guaranteed bythe outside vertical steel bracings during the next earthquake. With theoutside vertical steel bracings we don't gamble: we have a third line ofdefense (life saver).

[0143] A reinforced concrete building cannot provide this insurance.

[0144] (4) The system has a good behavior resisting the earthquakebecause the combination of reinforced concrete rigid frames and theoutside vertical steel bracings is improving the overall damping of theconcrete building, due to stub connectors.

[0145] (5) The design can be done by desk calculators. Accuracy indesigning the reinforced concrete rigid frames is not longer verycritical, since any excess earthquake force can be easily absorbed bythe outside vertical steel bracings.

[0146] (6) The outside vertical steel bracings provide a betterenvironment for the people living in the reinforced concrete buildingsbecause people have a feeling of comfort, knowing they have a betterchance to survive a severe earthquake.

[0147] Regarding Advantages No. 1 and No. 2, I want to mention andclarify the “elastic behavior,” the “failure stage,” and thedistribution of earthquake forces between reinforced concrete rigidframes and outside vertical steel bracings based on their absolutelateral stiffnesses.

[0148] In order to clarify these items, I will make a reference to areinforced concrete building analyzed by John J. Driskell and H. C. fromJohn J. Driskell & Associates, California, and U.C.L.A. extension (ACI)seminar, in December 1966.

[0149] The building has 30 stories, is 366 feet in height and in plan is80 feet by 180 feet.

[0150] On the outside of this concrete building, I am providing thevertical steel bracings.

[0151] The configuration in plan of the outside vertical steel bracingsis shown in FIGS. 6A through 7.

[0152] For the outside vertical steel bracings, I am studying twoalternates:

[0153] Alternate No. 1—All members of outside vertical steel bracings(columns, diagonals and struts) are W 14×730. This is a conservativesolution.

[0154] Alternate No. 2—The members of outside vertical steel bracingsare as follows:

[0155] Columns W 14×314

[0156] Diagonals W 14×87

[0157] Struts W 14×87

[0158] This is an economical solution.

Alternate No. 1

[0159] In order to find the distribution of forces, we must find theabsolute lateral stiffness of the reinforced concrete frame and outsidevertical steel bracings.

[0160] The Reinforced Concrete Frame

[0161] By applying a force H=250.Kips at the roof, we can calculate thetotal displacement of the roof relative to the base:$\Delta_{Roof} = {\Sigma \frac{Mm}{EI}\Delta_{s}}$

[0162] where the summation is for all members, columns and beams, fromfirst story to the roof. We get: Δ_(Roof) = 8.60  inches

[0163] and the absolute lateral stiffness:$\quad^{K}{Frame} = {\frac{H}{\Delta_{Roof}} = {\frac{250 \cdot K}{8.60\quad {Inches}} = {29 \cdot \frac{Kips}{Inch}}}}$

[0164] per one reinforced concrete frame.

[0165] The Outside Vertical Steel Bracings

[0166] By applying a force H=250.Kips at the roof, we can calculate thetotal displacement of the roof relative to the base:$\Delta_{Roof} = {\Sigma \frac{Nn}{EA}\Delta_{s}}$

[0167] where the summation is for all members, (columns, diagonals andstruts) from the first story to the roof. We get:Δ_(Roof) = 0.93  inches

[0168] and the absolute lateral stiffness:$\overset{K}{\quad}{{{vertical}\quad {bracings}} = {\frac{H}{\Delta_{Roof}} = {\frac{250 \cdot K}{0.93\quad {Inches}} = {269 \cdot \frac{Kips}{inch}}}}}\quad$

[0169] per one vertical steel bracing. Based on these stiffnesses, theearthquake forces will be resisted by the outside vertical steelbracings and the reinforced concrete frames. About 75% of earthquakeforces will be resisted by outside vertical steel bracings and about 25%will be resisted by the concrete rigid frames during the “elasticbehavior” based on absolute lateral stiffness.

[0170] The UBC Code also requires that the vertical steel bracings mustresist the total earthquake force. I call this criteria the “failurestage.”

[0171] In this Alternate No. 1, the outside vertical steel bracings areoverdesigned. Because of this overdesign, during the “elastic behavior,”the outside vertical steel bracings absorb a tremendouspercentage—75%—of the earthquake force.

[0172] During the “failure stage,” the outside vertical steel bracingswhich resist all the earthquake force are stressed to less than halfcapacity because the members are overdesigned.

[0173] This alternate is not an economical solution.

Alternate No. 2

[0174] Following the same procedure as in Alternate No. 1, we get:

[0175] The Reinforced Concrete Frame${{\,^{K}{Frame}} = {29 \cdot \frac{Kips}{inch}}},$

[0176] per one reinforced concrete frame.

[0177] This is the same value as in Alternate No. 1 because we did notchange the frame.

[0178] The Outside Vertical Steel Bracings $\begin{matrix}{\,^{K}{vertical}} \\{\quad {bracings}}\end{matrix} = {77 \cdot \frac{Kips}{inch}}$

[0179] per one vertical steel bracing.

[0180] This is a smaller value than in Alternate No. 1 because the sizesof steel members are smaller.

[0181] Based on these absolute lateral stiffnesses, the outside verticalsteel bracings resist about 50% of the earthquake forces and thereinforced concrete frames resist about 50% of the earthquake forcesduring the “elastic behavior.”

[0182] During the “failure stage,” the outside vertical steel bracingsmust resist the total earthquake forces. In Alternate No. 2, the steelmembers are stressed close to their capacity during the “failure stage”because the steel members have a smaller size the in Alternate No. 1.

[0183] This is an economical design. The outside vertical steel bracingsadd about 10% to the cost of the building, which is compensated by asimpler reinforcement for the concrete rigid frames.

Additional Comments

[0184] (1) Concrete floors must be designed as a diaphragm to transferhorizontal earthquake forces to the outside vertical steel bracings. Wemust design for shear (this may require the exterior bays to have athicker slab) and for top and bottom chords.

[0185] (2) Because of outside vertical steel bracings, all members mustbe designed for 1.25 times-earthquake forces (U.B.C. requirement forbraced frames).

[0186] (3) We must design the stub connectors which connect thereinforced concrete building to the outside vertical steel bracings.

[0187] (4) It is my belief that the reinforced concrete columns musthave at the first story only diagonal ties at 45 degrees in bothdirections on each face of the column. These ties at 45 degrees are moreefficient than a spiral reinforcement (see FIG. 6C). This system of tiesat 45 degrees can be tested in the laboratory to find out how efficientthey are.

[0188] In conclusion, this system of outside vertical steel bracings isespecially important in countries like U.S.A., Turkey, Yugoslavia, Iran,South American and China where the majority of the buildings are builtwith reinforced concrete because of economical reasons.

[0189] The system I am proposing can make the life of the reinforcedconcrete buildings longer and save the life of the people living andworking in these buildings.

[0190] {circle over (7.)} Interior Diagonal Bracing—Refinforced ConcreteStructures

[0191] The interior diagonal bracing for reinforced concrete structuresis similar to that described previously for steel structures (see FIGS.7A, 7B).

[0192] The outside vertical bracing system has excellent ductility underelastic load reversals (i.e., wind and moderate seismic forces). Thisproperty is inherent in the proposed system due to the elastic hinges(i.e., soft connections).

[0193] Elastic Hinge No. 1 is stub connector Nos. 1, 2, 3, and 4.

[0194] Elastic Hinge No. 2 is the point of contact between the column(interior or exterior column) and the diaphragm floor and the floatingfloor.

[0195] Elastic Hinge No. 3 is the column splice (flange plates and webplate).

[0196] (For location of Elastic Hinges No. 1, Soft Connections, see FIG.1 through FIG. 3.

[0197] Under inelastic load reversals (i.e., severe seismic forces), theelastic hinges have the potential of becoming plastic hinges, thusproviding an excellent ductility. The elastic hinges and the plastichinges act as structural fuses protecting the framing.

[0198] Another characteristic of the system is its ability to produceeffective damping action. This results from the division of the framingof the building into two sub-frames.

[0199] Sub-frame No. 1 consists of all of the diaphragm floors and theoutside vertical bracing. The diaphragm floors and the outside verticalbracing are connected through stub connectors Nos. 1, 2, 3 and 4.

[0200] Sub-frame No. 2 consits of all of the floors supported betweenthe diaphragm floors (floating floors) and all of the columns of thebuilding (interior and exterior). Sub-frames Nos. 1 and 2 are connectedwith elastic hinges which can develop into plastic hinges, insuringductility in response to a wide range of load intensities.

[0201] As a result of the design (interconnection) of Subframes Nos. 1and 2, wind and seismic forces are transferred from the building to thefoundations, or vice-versa, in a series of three motions.

[0202] Motion No. 1—the bending of all the columns supported between thediaphragm floors, transfers these forces from the floating floors to thediaphgram floors. This motion includes the rotations of Elastic HingesNos. 2 and 3 located on all the columns (interior and exterior).

[0203] Motion No. 2—the bending of stub connectors Nos. 1, 2, 3 and 4(Elastic Hinge No. 1) about the weak axis Y-Y, transfers the horizontalforces from the diaphgram floors to the outside vertical bracing(outside horizontal strut).

[0204] As these wind and seismic forces are transferred down to thefoundations, Motion No. 3 occurs. This consists of the bending of stubconnectors Nos. 1 and 4 (Elastic Hinges No. 1) about their strong axisX-X to transfer the overturning of the outside vertical bracing to theexterior columns D and G. These three motions impart a soft behavior tothe whole framing and result in a smaller base shear.

Key Words

[0205] Outside (Flying) Vertical Bracing, Elastic Hinges (SoftConnections), Plastic Hinges, Sub-frames, Damping, Diaphgram andFloating Floors, Soft Behavior, Ductility.

BRIEF DESCRIPTION OF THE DRAWINGS

[0206]FIG. 1 Generalized Structure

[0207]FIG. 1A Building Drift

[0208]FIG. 2 Framing Detail—Exterior Wall

[0209]FIG. 3 Member Loading—Seismic Case

[0210]FIG. 3A Exterior Elevation

[0211]FIG. 3B Interior Bracing—Sections

[0212]FIG. 4 Isometric elevation. Structural model of the whole framingand outside vertical bracing.

[0213]FIG. 5 Deflected shape of the column (interior or exterior)between diaphragm floors, under seismic or wind forces.

[0214]FIG. 6A Isometric elevation outside vertical steel bracingsconnected to a concrete building.

[0215]FIG. 6B Plan of Concrete Building with Steel Exterior Bracing

[0216]FIG. 6C Ties at 45 degrees in the reforced concrete columns at 1ststory only.

[0217]FIG. 7 Reinforced Concrete Building with Steel Exterior Bracing

[0218]FIG. 7A Reinforced Concrete Building with Steel Bracing

[0219]FIG. 7B Concrete Building—Section at Interior Bracing

[0220]FIG. 8 Spectrum of Horizontal Response

[0221]FIG. 9 Interior Structure

[0222]FIG. 10 Stability Structure

[0223]FIG. 11 Location of Anti-Seismic Springs

[0224]FIG. 12 Deflection of Stability Structure

[0225]FIG. 13 Deflection of the Interior Structure

[0226]FIG. 14 Axial Forces in Tons on Columns and Bracings of StabilityStructure

[0227]FIG. 15 Axial Forces in Tons and Moments in TM or corner columnsof Interior Structures

[0228]FIG. 16 Stresses in Springs (Tons)

[0229]FIG. 17 Exterior Elevation 1

[0230]FIG. 18 Exterior Elevation 2

[0231]FIG. 19 Exterior Elevation 3

[0232]FIG. 20 Is a partial perspective view of a building employing abracing system of the present invention.

[0233]FIG. 21 Is a close up view of the bracing systems of FIG. 20.

[0234]FIG. 22 Is a top plan view of the building of FIG. 20.

I claim:
 1. A system for reinforcing tall buildings comprising aninternal frame comprising a plurality of horizontal floor beams and aplurality of vertical columns extending between adjacent floor beams,the system comprising diagonal bracing members connected betweenvertically spaced floor beams at an angle to both the floor beams andthe columns.